Did settings of mixer change?
Did you check them?
Could it be simply disabled microphone?
I did step on this shovel myself before.
If you use gnome - try recorder application. It may give you hint as well sometimes.

I mean if nothing changed but kernel - something wrong with kernel then. Either you report bug to gentoo or to kernel devs.
I did communicate to alsa devs in past, they were nice to deal with._________________Just a user.